In this discussion of prior art, reference will be made to the following articles:
1. Wolfgang Jaschinski-Kruza in "Eyestrain in VDU Users: Viewing Distance and the Resting Position of Ocular Muscles" in Human Factors, 1991, 33(1), 69-83. PA1 2. D. Alfred Owens, "The Resting State of the Eyes" (1984) American Scientist, 72, 378-387. PA1 3. Wolfgang Jaschinski-Kruza, "Effects of Stimulus Distance on Measurements of Dark Convergence" Ophthalmic and Physiological Optics, 10, 243-251. PA1 (a) displaying a first one of the images at a predetermined intensity on the video display unit so as to be seen by a first eye of an observer only, PA1 (b) suppressing any after image of the first image to a second eye of the observer, and PA1 (c) displaying a second one of the images on the video display unit so as to be seen by the second eye of the observer only at said predetermined intensity. PA1 (a) measuring an interpupillary distance of the observer, PA1 (b) determining a visual threshold of the observer, PA1 (c) presenting a series of two successive images on a video display unit as a dichoptic stimulus of substantially equal intensity such that a first one of the images is stationary and a second one of the images may be moved by illuminating different pixels of the video display unit, PA1 (d) for each series of images presented on the video display unit, moving the second one of the images until both images appear to the observer to be aligned, PA1 (e) measuring any actual displacement between the two images, and PA1 (f) calculating a distance of dark vergence according to the formula: ##EQU1## where: D.sub.c is the distance of dark vergence;
It was long thought that, when at rest, a person's eyes are focused at infinity. That this view has now been shown to be erroneous is discussed at length by Jaschinski-Kruza.sup.1 who shows that, contrary to the traditional view cited above, intermediate positions of accommodation and convergence are generally observed in a visually featureless environment.
D. Alfred Owens.sup.2 shows that visual perception under adverse conditions is, to some extent, dependent on the natural tonus of the eye muscles whereupon whenever visual conditions are degraded, the eyes tend to shift involuntarily to the individual's "resting" distance.
As is well known, the convexity of the crystalline lens on the front of the eye is adjusted by the ciliary muscles in order to focus on objects at different distances from the eye. At the same time, the eyes' axes are rotated by the extraocular muscles so that a person's eyes are directed towards an object of interest. The closer the object of interest is to the eyes of the observer, the greater is the required rotation of the eyes of the observer by the extraocular muscles. It was once assumed that, since minimum rotation is required when the eye is focused at infinity, viewing any object closer than infinity strains the extraocular muscles thereby causing eyestrain and related fatigue.
However, as reported by D. Alfred Owens.sup.2 and Jaschinski-Kruza.sup.1,3, in fact, when most subjects look into a homogeneous field without any fixation stimulus, e.g. into darkness, their visual axes converge to a certain intermediate distance. This distance is variably referred to by the terms "dark convergence", "tonic vergence", "resting position of convergence" and is referred to throughout the remainder of this specification and claims under the term "dark vergence".
Jaschinski-Kruza.sup.1 reports that the more distant a subject's dark vergence, the greater is the visual strain at a 50 cm viewing distance. Specifically, a marked difference between the distance of a subject to a VDU from the characteristic dark vergence of the subject, the more pronounced will be the resulting operator fatigue consequent to extended use of the VDU. This having been shown to be the case, it is clearly desirable to adjust, as far as possible, an operator's distance from a VDU so as to be equal to the dark vergence of the operator. D. Alfred Owens.sup.2 measured the dark vergence of 220 college students all of whom either did not require correction spectacles or were wearing their normal spectacles and were told to "relax" their eyes while their accommodation was measured in total darkness. On average, subjects focused for a distance of approximately 67 cm in the dark. While a few (about 1%) focused to optical infinity, as predicted by classical theory, others focused as close as 25 cm.
Jaschinski-Kruza.sup.1 shows the correlation between variation in viewing distance from an operator's dark vergence and operator fatigue. When the subjects were free to adjust the viewing distance, they were most comfortable when the distances thus chosen were between 51 and 99 cm with characters 5 mm tall. This correlates well with the measured average dark vergence of 67 cm reported by D. Alfred Owens.sup.2.
From the foregoing, two things are clear: first, dark vergence varies between limits from one subject to another; and the optimal comfort distance from an operator to a video display unit is influenced by the operator's dark vergence. Thus, the question arises as to how dark vergence may be measured for a particular subject.
As reported by Jaschinski-Kruza.sup.3, dark vergence is typically measured psychophysically by aligning two dichoptic stimuli which are briefly flashed in a dark surround. Jaschinski-Kruza.sup.3 reports the effect of varying the viewing distance of the stimulus and further reports that the measured dark vergence was biased towards the actual viewing distance of the stimuli: this effect being stronger with bar stimuli than with point-and-line stimuli. The subject's foreknowledge of the viewing distance had a small but nevertheless significant effect; whilst the size of the nonius bars had none. The method employed by Jaschinski-Kruza is based on the coincidence of nonius bars whereby two vertical light bars are presented dichoptically on a dark surround, dichoptic separation being achieved by red and green filters. The bars were exposed for 100 ms every 2.5 s, the mutual separation between the bars being adjusted in a staircase procedure until perceived by the observers as being vertically aligned.
According to an alternative method, a bright red light-emitting diode (LED) was flashed in a dark surround for 100 ms at 2.5 s intervals. A Maddox rod and a rotary prism were placed in front of the observer's left eye, the prism being rotated by the observer to a position wherein the red point seen by the right eye coincided with the vertical line of light visible through the Maddox rod in front of the left eye. Suitable measurements and calculations were then performed in order to determine the dark vergence of the subject.
Although the correlation between dark vergence and eyestrain have been clearly established and although known methods for determining dark vergence are based on the alignment of dichoptic stimuli, no simple instrument has been provided which permits the presentation of dichoptic stimuli on a VDU. The reason for this probably lies in the fact that, in order to present dichoptic stimuli to the eyes, each eye must be forced to see its own image and only its own image. If, to the contrary, one eye sees the image (even partially) which was presented to its fellow eye, then the whole basis of the psychophysical measurement is destroyed. Herein lies the problem with using a VDU to present dichoptic stimuli because, as is known, the pixels of which the VDU screen is composed, continue to phosphoresce even when they are no longer subjected to a source of electric power. Consequently, if a first image is displayed on a VDU by illuminating certain pixels thereof in order that only one eye can see the image and then the image is extinguished prior to illuminating other pixels, constituting a second image for viewing by the fellow eye, then the fellow eye will see not only its own image but will also see the after image of the first image owing to the residual phosphorescence of the pixels. If, in order to avoid this, a sufficiently long time period is allowed to elapse between the successive presentation of the successive images then, whilst the problem of residual after image will be avoided, the two images will no longer be dichoptic. In other words, the obvious solution to avoiding the effect of after image, namely to lengthen the time period between the presentation on the VDU of successive images, militates against the two images being seen by the eyes as a dichotic stimulus.
No solution to this problem has been reported, so far as is known, in either the scientific or the patent literature.